In Europe, the term PABX (Private Automatic Branch eXchange) is often used.
In industrial countries, most companies (with more than around 10 employes) need their employes to be able call each other, call outside phone numbers (the public telephone network or PSTN), and receive calls from outside.
For companies with multiple physical locations, PBXs are sometimes interconnected by so called trunk lines.
PBXs are distinguished from smaller "key systems[?]" by the fact that external lines are not normally indicated and selectable from an individual extension. From a user's point of view calls on a key system are made by selecting a line and dialing the external number; calls on a PBX are made by dialing 9 then the external number.
Another alternative is to connect all the telephone sets to the PSTN, but the major disandvantage is that every extension requires its own line (usually with a monthly recurring line charge); also, "internal" calls would be dialed externally, and charged for.
Finally, most local phone companies offer Centrex[?] or "Virtual PBX" service in which each extension has a trunk line are connected to the telephone company's Central Office[?], where software on the CO switch enables PBX-like functionality.
Functionally, the PBX performs three main duties: - Establishing connections (circuits) between the telephone sets of two users. Note that fax, modems and many communication devices can often be connected to the PBX (although the pbx may degrade line quality for modems). Therefore telephone sets are referred to as extensions. - Maintaining such connections as long as the users require them. - Providing information for the Accouting Department (e.g. metering calls)
PBXs offer many capabilities, although each manufacturer may have a different name for each capability. Here is a short list of common capabilities:
The extension interface can be:
The most common digital standard for fixed devices is ISDN.
Cordless phones can also be used on sofisticated PBX: DECT devices become a standard.
Potential links between PBX (trunk lines) can also use proprietary protocols, but if severals manufacters are on site, the use of a standard protocol is required. Most used standard protocols are QSIG and DPNSS.